Empirical & Molecular Formula
Lajoy Tucker
Teacher
Contents
Empirical Formula
The empirical formula of a compound represents the simplest whole-number ratio of atoms of each element in the compound.
For example, in glucose (), the empirical formula is , which represents the ratio of carbon to hydrogen to oxygen.
Empirical formula from elemental analysis
1. Determine the mass or percentage of each element in the sample.
2. Convert the mass of each element to moles using the formula:
3. Divide the number of moles of each element by the smallest number of moles to find the simplest ratio.
Worked Example
A compound is found to consist of g of Carbon and g of Hydrogen. Calculate the empirical formula.
Answer
Element | Carbon | Hydrogen |
|---|---|---|
Mass (or percentage) | ||
Moles | ||
Divide by smallest number of moles * |
the empirical formula is .
Remember the empirical formula is the simplest whole number ratio.
Some calculations will not immediately give a whole number and must be appropriately scaled.
No answer provided.
Worked Example
A salt X has the formula .
X is found to contain % , % and the remainder being oxygen.
Use the data above to calculate the values of and
Answer
Element | Aluminium | Sulfur | Oxygen |
Mass (or percentage) | |||
| |||
Moles | |||
Divide by smallest number of moles |
1: 1.5 : 4.5 is not a whole number ratio.
A while number ratio can be achieved by multiplying all values by 2
2:3:9
Therefore, the empirical formula is .
To align with the formula in the question , the final answer is where , and
Empirical formula from combustion analysis
The empirical formula of organic compounds can be determined through combustion in excess oxygen.
The masses or volumes of carbon dioxide () and water () produced can be directly related to the number of moles of carbon and hydrogen in the original compound.
General equation for complete combustion of a hydrocarbon
Where x and y represent the number of carbon and hydrogen atoms (respectively) in the original compound.
The simplest ratio can then be found by diving by the smallest number of moles (as before).
Worked Example
A hydrocarbon was burned in excess oxygen producing of carbon dioxide and of water. Determine the empirical formula.
Answer
Step 1: Determine moles of and from moles of and .
Moles
x = moles of = moles of in original compound
Therefore, moles of
Moles
= moles of and y = moles of H in original compound
Therefore, moles of
Step 2: Divide by the smallest moles to determine the simplest whole number ratio.
C | H | |
|---|---|---|
Moles | ||
Divide by the smallest |
Therefore, the empirical formula is
Molecular Formula
The molecular formula represents the actual number of atoms of each element in a compound
It may or may not be the same as the empirical formula
Steps to find the Molecular Formula:
1. Find the empirical formula as described previously.
2. Calculate the empirical formula mass (sum of the atomic masses of the atoms in the empirical formula).
3. Divide the relative formula mass of the compound by the empirical formula mass to find the multiplication factor.
4. Multiply the empirical formula by this factor to obtain the molecular formula.
Worked Example
A compound has an empirical formula of , and its is .
Calculate the molecular formula.
Answer
1. Calculate the empirical formula mass of = Simplest whole number ratio of atoms
2. Calculate the factor
3. Find the molecular formula = Each atom multiplied by the factor
Molecular formula=
Thus, the molecular formula is .
Practice Question
Question:
Analysis of a compound shows that it contains:
% by mass of Carbon
% by mass of Hydrogen
% by mass of Chlorine
1. Calculate the empirical formula of .
2. Use this empirical formula and the relative molecular mass of to calculate the molecular formula of .
Answer
Part 1
Empirical formula of :
1. Divide the % of by the to find moles
Moles of Carbon
Moles of Hydrogen
Moles of Chlorine
2. Divide by the smallest moles to find the ratio of atoms within the compound
Smallest moles
Carbon
Hydrogen
Chlorine
Therefore, the empirical formula of
Part 2
1. Find the relative formula mass of the empirical formula
The relative formula mass of the empirical formula
= Relative formula mass
= Relative formula mass
= Relative formula mass
2. Divide the relative formula mass of by the relative formula mass of the empirical formula
Ratio between of and the of the empirical formula
Ratio between of and the of the empirical formula
3. Finally, multiply each number of elements by
Therefore, the molecular formula of is